Field of the Invention
The present invention relates to the biochemical arts, in particular to therapeutic peptides and conjugates.
Discussion of the Related Art
Voltage-gated sodium channels (VGSC) are glycoprotein complexes responsible for initiation and propagation of action potentials in excitable cells such as central and peripheral neurons, cardiac and skeletal muscle myocytes, and neuroendocrine cells. Mammalian sodium channels are heterotrimers, composed of a central, pore-forming alpha (α) subunit and auxiliary beta (β) subunits. Mutations in alpha subunit genes have been linked to paroxysmal disorders such as epilepsy, long QT syndrome, and hyperkalemic periodic paralysis in humans, and motor endplate disease and cerebellar ataxia in mice. (Isom, Sodium channel beta subunits: anything but auxiliary, Neuroscientist 7(1):42-54 (2001)). The β-subunit modulates the localization, expression and functional properties of α-subunits in VGSCs.
Voltage gated sodium channels comprise a family consisting of 9 different subtypes (NaV1.1-NaV1.9). As shown in Table 1, these subtypes show tissue specific localization and functional differences (See, Goldin, A. L., Resurgence of sodium channel research, Annu Rev Physiol 63: 871-94 (2001); Wilson et al., Compositions useful as inhibitors of voltage-gated ion channels, US 2005/0187217 A1). Three members of the gene family (NaV1.8, 1.9, 1.5) are resistant to block by the well-known sodium channel blocker tetrodotoxin (TTX), demonstrating subtype specificity within this gene family. Mutational analysis has identified glutamate 387 as a critical residue for TTX binding (See, Noda, M., H. Suzuki, et al., A single point mutation confers tetrodotoxin and saxitoxin insensitivity on the sodium channel II″ FEBS Lett 259(1): 213-6 (1989)).
TABLE 1VGSC family with rat TTX IC50 values.TTXVGSCIC50isoformTissue(nM)IndicationNav1.1CNS, PNS10Pain, Epilepsy,soma ofNeurodegenerationneuronsNav1.2CNS10Neurodegeneration, Epilepsyhigh in axonsNav1.3CNS,2-15Pain, Epilepsyembryonic,injured nervesNav1.4Skeletal muscle5MyotoniaNav1.5heart2000Arrhythmia, long QTNav1.6CNS1Pain, movement disorderswidespread, mostabundantNav1.7PNS, DRG,4Pain, Neuroendocrineterminalsdisorders, prostate cancerneuroendocrineNav1.8PNS, small neurons>50,000Painin DRG & TGNav1.9PNS, small neurons1000Painin DRG & TGAbbreviations: CNS = central nervous system, PNS = peripheral nervous system, DRG = dorsal root ganglion, TG = Trigeminal ganglion. (See, Wilson et al., Compositions useful as inhibitors of Voltage-gated ion channels, US 2005/0187217 A1; Goldin, Resurgence of Sodium Channel Research, Annu Rev Physiol 63: 871-94 (2001)).
In general, voltage-gated sodium channels (Nays) are responsible for initiating the rapid upstroke of action potentials in excitable tissue in the nervous system, which transmit the electrical signals that compose and encode normal and aberrant pain sensations. Antagonists of NaV channels can attenuate these pain signals and are useful for treating a variety of pain conditions, including but not limited to acute, chronic, inflammatory, and neuropathic pain. Known NaV antagonists, such as TTX, lidocaine, bupivacaine, phenytoin, lamotrigine, and carbamazepine, have been shown to be useful for attenuating pain in humans and animal models. (See, Mao, J. and L. L. Chen, Systemic lidocaine for neuropathic pain relief, Pain 87(1): 7-17 (2000); Jensen, T. S., Anticonvulsants in neuropathic pain: rationale and clinical evidence, Eur J Pain 6 (Suppl A): 61-68 (2002); Rozen, T. D., Antiepileptic drugs in the management of cluster headache and trigeminal neuralgia, Headache 41 Suppl 1: S25-32 (2001); Backonja, M. M., Use of anticonvulsants for treatment of neuropathic pain, Neurology 59(5 Suppl 2): S14-7 (2002)).
The α-subunits of TTX-sensitive, voltage-gated NaV1.7 channels are encoded by the SCN9A gene. The NaV1.7 channels are preferentially expressed in peripheral sensory neurons of the dorsal root ganglia, some of which are involved in the perception of pain. In humans, mutations in the SCN9A gene have shown a critical role for this gene in pain pathways. For instance, a role for the NaV1.7 channel in pain perception was established by recent clinical gene-linkage analyses that revealed gain-of-function mutations in the SCN9A gene as the etiological basis of inherited pain syndromes such as primary erythermalgia (PE), inherited erythromelalgia (IEM), and paroxysmal extreme pain disorder (PEPD). (See, e.g., Yang et al., Mutations in SCN9A, encoding a sodium channel alpha subunit, in patients with primary erythermalgia, J. Med. Genet. 41:171-174 (2004); Harty et al., NaV1.7 mutant A863P in erythromelalgia: effects of altered activation and steady-state inactivation on excitability of nociceptive dorsal root ganglion neurons, J. Neurosci. 26(48):12566-75 (2006); Estacion et al., NaV1.7 gain-of-function mutations as a continuum: A1632E displays physiological changes associated with erythromelalgia and paroxysmal extreme pain disorder mutations and produces symptoms of both disorders, J. Neurosci. 28(43):11079-88 (2008)). In addition, overexpression of NaV1.7 has been detected in strongly metastatic prostate cancer cell lines. (Diss et al., A potential novel marker for human prostate cancer: voltage-gated sodium channel expression in vivo, Prostate Cancer and Prostatic Diseases 8:266-73 (2005); Uysal-Onganer et al., Epidermal growth factor potentiates in vitro metastatic behavior human prostate cancer PC-3M cells: involvement of voltage-gated sodium channel, Molec. Cancer 6:76 (2007)).
Loss-of-function mutations of the SCN9A gene result in a complete inability of an otherwise healthy individual to sense any form of pain. (e g, Ahmad et al., A stop codon mutation in SCN9A causes lack of pain sensation, Hum. Mol. Genet. 16(17):2114-21 (2007)).
A cell-specific deletion of the SCN9A gene in conditional knockout mice reduces their ability to perceive mechanical, thermal or inflammatory pain. (Nassar et al., Nociceptor-specific gene deletion reveals a major role for NaV1.7 (PN1) in acute and inflammatory pain, Proc. Natl. Acad. Sci, USA. 101(34): 12706-12711 (2004)).
Based on such evidence, decreasing NaV1.7 channel activity or expression levels in peripheral sensory neurons of the dorsal root ganglia (DRG) has been proposed as an effective pain treatment, e.g. for chronic pain, neuropathic pain, and neuralgia. (E.g., Thakker et al., Suppression of SCN9A gene expression and/or function for the treatment of pain, WO 2009/033027 A2; Yeomans et al., Decrease in inflammatory hyperalgesia by herpes vector-mediated knockdown of NaV1.7 sodium channels in primary afferents, Hum. Gene Ther. 16(2):271-7 (2005); Fraser et al., Potent and selective NaV1.7 sodium channel blockers, WO 2007/109324 A2; Hoyt et al., Discovery of a novel class of benzazepinone Na(v)1.7 blockers: potential treatments for neuropathic pain, Bioorg. Med. Chem. Lett. 17(16):4630-34 (2007); Hoyt et al., Benzazepinone NaV1.7 blockers: Potential treatments for neuropathic pain, Bioorg. Med. Chem. Lett. 17(22):6172-77 (2007)).
The α-subunits of TTX-sensitive, voltage-gated NaV1.3 channels are encoded by the SCN3A gene. Four splice variants of human Nav1.3 were reported to have different biophysical properties. (Thimmapaya et al., Distribution and functional characterization of human NaV1.3 splice variants, Eur. J. Neurosci. 22:1-9 (2005)). Expression of NaV1.3 has been shown to be upregulated within DRG neurons following nerve injury and in thalamic neurons following spinal cord injury. (Hains et al., Changes in electrophysiological properties and sodium channel NaV1.3 expression in thalamic neurons after spinal cord injury, Brain 128:2359-71 (2005)). A gain-in-function mutation in Nav1.3 (K354Q) was reportedly linked to epilepsy. (Estacion et al., A sodium channel mutation linked to epilepsy increases ramp and persistent current of NaV1.3 and induces hyperexcitability in hippocampal neurons, Experimental Neurology 224(2):362-368 (2010)).
Toxin peptides produced by a variety of organisms have evolved to target ion channels. Snakes, scorpions, spiders, bees, snails and sea anemones are a few examples of organisms that produce venom that can serve as a rich source of small bioactive toxin peptides or “toxins” that potently and selectively target ion channels and receptors. In most cases, these toxin peptides have evolved as potent antagonists or inhibitors of ion channels, by binding to the channel pore and physically blocking the ion conduction pathway. In some other cases, as with some of the tarantula toxin peptides, the peptide is found to antagonize channel function by binding to a region outside the pore (e.g., the voltage sensor domain).
Native toxin peptides are usually between about 20 and about 80 amino acids in length, contain 2-5 disulfide linkages and form a very compact structure. Toxin peptides (e.g., from the venom of scorpions, sea anemones and cone snails) have been isolated and characterized for their impact on ion channels. Such peptides appear to have evolved from a relatively small number of structural frameworks that are particularly well suited to addressing the critical issues of potency, stability, and selectivity. (See, e.g., Dauplais et al., On the convergent evolution of animal toxins: conservation of a diad of functional residues in potassium channel-blocking toxins with unrelated structures, J. Biol. Chem. 272(7):4302-09 (1997); Alessandri-Haber et al., Mapping the functional anatomy of BgK on Kv1.1, Kv1.2, and Kv1.3, J. Biol. Chem. 274(50):35653-61 (1999)). The majority of scorpion and Conus toxin peptides, for example, contain 10-40 amino acids and up to five disulfide bonds, forming extremely compact and constrained structures (microproteins) often resistant to proteolysis. The conotoxin and scorpion toxin peptides can be divided into a number of superfamilies based on their disulfide connections and peptide folds. The solution structure of many of these has been determined by Nuclear Magnetic Resonance (NMR) spectroscopy, illustrating their compact structure and verifying conservation of their family folding patterns. (E.g., Tudor et al., Ionisation behaviour and solution properties of the potassium-channel blocker ShK toxin, Eur. J. Biochem. 251(1-2):133-41(1998); Pennington et al., Role of disulfide bonds in the structure and potassium channel blocking activity of ShK toxin, Biochem. 38(44): 14549-58 (1999); Jaravine et al., Three-dimensional structure of toxin OSK1 from Orthochirus scrobiculosus scorpion venom, Biochem. 36(6):1223-32 (1997); del Rio-Portillo et al.; NMR solution structure of Cn12, a novel peptide from the Mexican scorpion Centruroides noxius with a typical beta-toxin sequence but with alpha-like physiological activity, Eur. J. Biochem. 271(12): 2504-16 (2004); Prochnicka-Chalufour et al., Solution structure of discrepin, a new K+-channel blocking peptide from the alpha-KTx15 subfamily, Biochem. 45(6):1795-1804 (2006)). Conserved disulfide structures can also reflect the individual pharmacological activity of the toxin family. (Nicke et al. (2004), Eur. J. Biochem. 271: 2305-19, Table 1; Adams (1999), Drug Develop. Res. 46: 219-34). For example, α-conotoxins have well-defined four cysteine/two disulfide loop structures (Loughnan, 2004) and inhibit nicotinic acetylcholine receptors. In contrast, ω-conotoxins have six cysteine/three disulfide loop consensus structures (Nielsen, 2000) and block calcium channels. Structural subsets of toxins have evolved to inhibit either voltage-gated or calcium-activated potassium channels.
Spider venoms contain many peptide toxins that target voltage-gated ion channels, including Kv, Cav, and Nay channels. A number of these peptides are gating modifiers that conform to the inhibitory cystine knot (ICK) structural motif. (See, Norton et al., The cystine knot structure of ion channel toxins and related polypeptides, Toxicon 36(11):1573-1583 (1998); Pallaghy et al., A common structural motif incorporating a cystine knot and a triple-stranded β-sheet in toxic and inhibitory polypeptides, Prot. Sci. 3(10):1833-6, (1994)). In contrast to some scorpion and sea anemone toxins, many spider toxins do not affect the rate of inactivation but inhibit channel activity by restricting the movement of the voltage sensor into the open channel conformation, shifting their voltage dependence of activation to a more positive potential. Many of these spider toxins are promiscuous within and across voltage-gated ion channel families.
A variety of toxin peptides that target VGSCs, in particular, have been reported. (See, Billen et al., Animal peptides targeting voltage-activated sodium channels, Cur. Pharm. Des. 14:2492-2502, (2008)). Three classes of peptide toxins have been described: 1) site 1 toxins, the μ-conotoxins, bind to the pore of the channel and physically occlude the conduction pathway; 2) site 3 toxins, including the α-scorpion toxins, some sea anemone toxins and δ-conotoxins, bind to the S3-S4 linker of domain IV and slow channel inactivation; and 3) site 4 toxins, including the β-scorpion toxins, bind to the S3-S4 linker in domain II and facilitate channel activation. Both site 3 and site 4 families of peptides alter the open probability of NaV channels and affect gating transitions and are therefore called “gating modifiers.”
μ-Conotoxin KIIIA (SEQ ID NO:530), a site 1 toxin originally isolated from Conus kinoshitai, is a C-terminally amidated peptide 16 amino acids in length that contains 6 cysteine residues engaged in 3 intramolecular disulfide bonds. It was initially characterized as an inhibitor of tetrodotoxin (TTX)-resistant sodium channels in amphibian dorsal root ganglion (DRG) neurons. (See, Bulaj et al., Novel conotoxins from Conus striatus and Conus kinoshitai selectively block TTX-resistant sodium channels, Biochem. 44(19):7259-7265, (2005)). Later it was found to more effectively inhibit TTX-sensitive than TTX-resistant sodium current in mouse DRG neurons. (See, Zhang et al., Structure/function characterization of μ-conotoxin KIIIA, an analgesic, nearly irreversible blocker of mammalian neuronal sodium channels, J. Biol. Chem. 282(42):30699-30706, (2007)) KIIIA has been found to block cloned mammalian (rodent) channels expressed in Xenopus laevis oocytes with the following rank order potency: NaV1.2>NaV1.4>NaV1.6>NaV1.7>NaV1.3>NaV1.5. Intraperitoneal injection of KIIIA has demonstrated analgesic activity in a formalin-induced pain assay in mice with an ED50 of 1.6 nmol/mouse (0.1 mg/kg) without observed motor impairment; some motor impairment but not paralytic activity was observed at a higher dose (10 nmol). (See, Zhang et al., 2007). Substitution of alanine for Lys7 and Arg10 modified maximal block, while substitution of His12 and Arg14 altered Nav isoform specificity. (See, McArthur et al., Interactions of key charged residues contributing to selective block of neuronal sodium channels by μ-conotoxin KIIIA, Mol. Pharm. 80(4): 573-584, (2011)). “Alanine scan” analogs of KIIIA have identified Lys7, Trp8, Arg10, Asp11, His12, and Arg14 as being important for activity against rNaV1.4. (See Zhang et al., 2007). The NMR solution structure of KIIIA places these residues within or adjacent to an α-helix near the C-terminus of the molecule. (See, Khoo et al., Structure of the analgesic μ-conotoxin KIIIA and effects on the structure and function of disulfide deletion, Biochem. 48(6):1210-1219, (2009)). The disulfide bond between Cys1 and Cys9 may be removed by substitution of alanine (KIIIA[C1A,C9A]) without greatly reducing the activity of the compound. (See, Khoo et al., 2009; Han et al., Structurally minimized μ-conotoxin analogs as sodium channel blockers: implications for designing conopeptide-based therapeutics, ChemMedChem 4(3):406-414, (2009)). Replacing a second disulfide bond between Cys2 and Cys16 with a diselenide bond between selenocysteine residues has given rise to the disulfide-depleted selenoconopeptide analogs of KIIIA These compounds have retained the activity of KIIIA but are more synthetically accessible. (See, Han et al., Disulfide-depleted selenoconopeptides: simplified oxidative folding of cysteine-rich peptides, ACS Med. Chem. Lett. 1(4):140-144, (2010)). The native structure has been further minimized to a lactam-stabilized helical peptide scaffold with Nay inhibitory activity. (See, Khoo et al., Lactam-stabilized helical analogues of the analgesic μ-conotoxin KIIIA, J. Med. Chem. 54:7558-7566 (2011)) KIIIA binds to the neurotoxin receptor site 1 in the outer vestibule of the conducting pore of the VGSCs and blocks the channel in an all-or-none manner. Recent studies have shown that some analogs of KIIIA only partially inhibit the sodium current and may be able to bind simultaneously with TTX and saxitoxin (STX). (See, Zhang et al., Cooccupancy of the outer vestibule of voltage-gated sodium channels by μ-conotoxin KIIIA and saxitoxin or tetrodotoxin, J. Neurophys. 104(1):88-97, (2010); French et al., The tetrodotoxin receptor of voltage-gated sodium channels—perspectives from interactions with μ-conotoxins, Marine Drugs 8:2153-2161, (2010); Zhang et al., μ-Conotoxin KIIIA derivatives with divergent affinities versus efficacies in blocking voltage-gated sodium channels. Biochem. 49(23):4804-4812, (2010); Zhang et al., Synergistic and antagonistic interactions between tetrodotoxin and μ-conotoxin in blocking voltage-gated sodium channels, Channels 3(1):32-38, (2009)).
OD1 (SEQ ID NO:589) is an α-like toxin isolated from the venom of the Iranian yellow scorpion Odonthobuthus doriae. (See, Jalali et al., OD1, the first toxin isolated from the venom of the scorpion Odonthobuthus doriae active on voltage-gated Na+ channels, FEBS Lett. 579(19):4181-4186, (2005)). This peptide is 65 amino acids in length with an amidated C-terminus containing 6 cysteine residues that form 3 disulfide bonds. OD 1 has been characterized as an NaV1.7 modulator that impairs fast inactivation (EC50=4.5 nM), increases the peak current at all voltages, and induces a persistent current, with selectivity against NaV1.8 and NaV1.3. (See Maertens et al., Potent modulation of the voltage-gated sodium channel Nav1.7 by OD1, a toxin from the scorpion Odonthobuthus doriae, Mol. Pharm. 70(1):405-414, (2006)).
Huwentoxin-IV(HWTX-IV; SEQ ID NO:528) is a 35 residue C-terminal peptide amide with 3 disulfide bridges between 6 cysteine residues isolated from the venom of the Chinese bird spider, Selenocosmia huwena. (See, Peng et al., Function and solution structure of huwentoxin-IV, a potent neuronal tetrodotoxin (TTX)-sensitive sodium channel antagonist from chinese bird spider Selenocosmia huwena, J. Biol. Chem. 277(49):47564-47571, (2002)). The disulfide bonding pattern (C1-C4, C2-5, C3-C6) and NMR solution structure place HWTX-IV in the ICK structural family since the C3-C6 disulfide bond passes through the 16-residue ring formed by the other two disulfide bridges (C1-C4 and C2-C5). HWTX-IV inhibits TTX-sensitive sodium currents in adult rat DRG neurons with an IC50 value of 30 nM but has no effect on TTX-resistant VGSCs at up to a 100 nM concentration. (See, Peng et al., 2002). HWTX-IV was also 12-fold less potent against central neuronal sodium channels in rat hippocampus neurons, suggesting that it may be selective toward NaV1.7. (See, Xiao et al., Synthesis and characterization of huwentoxin-IV, a neurotoxin inhibiting central neuronal sodium channels, Toxicon 51(2):230-239, (2008)). Testing HWTX-IV against VGSC sub-types determined the relative sensitivity to be hNav1.7 (IC50=26 nM)>rNav1.2>>rNav1.3>rNav1.4>hNav1.5. (See Xiao et al., Tarantula huwentoxin-IV inhibits neuronal sodium channels by binding to receptor site 4 and trapping the domain II voltage sensor in the closed configuration, J. Biol. Chem. 283(40):27300-27313, (2008)). Site directed protein mutagenesis mapped the binding of HWTX-IV to neurotoxin site 4, the extracellular S3-S4 linker between domain II, and its behavior in response to changes in voltage and channel activation is consistent with binding to the voltage sensor of Nav1.7 and trapping it in the resting configuration. (See, Xiao et al., 2008). Huwentoxin-I (HWTX-I; SEQ ID NO:529), a related family member is less potent against VGSCs but is active against N-Type CaV channels. (See, Wang et al., The cross channel activities of spider neurotoxin huwentoxin I on rat dorsal root ganglion neurons, Biochem. Biophys. Res. Comm. 357(3):579-583, (2007); Chen et al., Antinociceptive effects of intrathecally administered huwentoxin-I, a selective N-type calcium channel blocker, in the formalin test in conscious rats, Toxicon 45(1):15-20, (2005); Liang et al., Properties and amino acid sequence of huwentoxin-I, a neurotoxin purified from the venom of the Chinese bird spider Selenocosmia huwena, Toxicon 31(8):969-78, (1993)).
ProTx-II (SEQ ID NO:531), isolated from the venom of the tarantula Thixopelma pruriens, is a 30 amino acid polypeptide with a C-terminal free acid and 6 cysteine residues that form 3 disulfide bonds. It differs from other members of the ICK family because it contains only three residues between the fifth and sixth cysteine residues instead of the normal 4-11. ProTx-II is a potent inhibitor of several NaV channel sub-types including NaV1.2, NaV1.7 (IC50<1 nM), NaV1.5, and NaV1.8, as well as Cav3.1 channels but not KV channels. (See, Middleton et al., Two tarantula peptides inhibit activation of multiple sodium channels, Biochem. 41(50):14734-14747, (2002); Priest et al., ProTx-I and ProTx-II: gating modifiers of voltage-gated sodium channels, Toxicon 49(2):194-201, (2007); Edgerton et al., Inhibition of the activation pathway of the T-type calcium channel CaV3.1 by ProTxII, Toxicon 56(4):624-636, (2010)). The “alanine scan” analogs of ProTx-II were tested against Nav1.5, identifying Meth, Trp7, Arg13, Met19, Val20, Arg22, Leu23, Trp24, Lys27, Leu29, and Trp30 as being important for activity. (See, Smith et al., Molecular interactions of the gating modifier toxin ProTx-II with Nav1.5: implied existence of a novel toxin binding site coupled to activation, J. Biol. Chem. 282(17):12687-12697, (2007)). Biophysical characterization showed that ProTx-II differs from HwTx-IV in its ability to interact with lipid membranes. (See, Smith et al., Differential phospholipid binding by site 3 and site 4 toxins: implications for structural variability between voltage-sensitive sodium channel comains, J. Biol. Chem. 280(12):11127-11133, (2005). Doses of 0.01 and 0.1 mg/kg i.v. of ProTx-II were well tolerated in rats, but 1 mg/kg doses were lethal. ProTx-II was not efficacious in a mechanical hyperalgesia study. (See, Schmalhofer et al., ProTx-II, a selective inhibitor of NaV1.7 sodium channels, blocks action potential propagation in nociceptors, Mol. Pharm. 74(5):1476-1484, (2008)). Intrathecal administration was lethal at 0.1 mg/kg and not effective in the hyperalgesia study at lower doses. ProTx-II application to desheathed cutaneous nerves completely blocked the C-fiber compound action potential but had little effect on action potential propagation of the intact nerve. (See, Schmalhofer et al., 2008). ProTx-II is believed to bind to the S3-S4 linker of domain II of NaV1.7 to inhibit channel activation but may also interact with the domain IV voltage sensor and affect sodium channel activation at higher concentrations. (See, Xiao et al., The tarantula toxins ProTx-II and huwentoxin-IV differentially interact with human Nav1.7 voltage sensors to inhibit channel activation and inactivation, Mol. Pharm. 78(6):1124-1134, (2010); Sokolov et al., Inhibition of sodium channel gating by trapping the domain II voltage sensor with protoxin II, Mol. Pharm. 73(3):1020-1028, (2008); Edgerton et al., Evidence for multiple effects of ProTxII on activation gating in NaV1.5, Toxicon 52(3):489-500, (2008)).
Other peptide inhibitors of VGSCs isolated from spider venoms and peptide analogs were reported in Murray et al., Potent and selective inhibitors of Nav1.3 and Nav1.7, WO 2012/125973 A2; Meir et al., Novel peptides isolated from spider venom and uses thereof, US 2011/0065647 A1; Lampe et al., Analgesic peptides from venom of Grammostola spatulata and use thereof, U.S. Pat. No. 5,877,026; Park et al., Analogs of sodium channel peptide toxin, WO 2012/004664 A2; and Chen et al., Molecular diversity and evolution of cysteine knot toxins of the tarantula Chilobrachys jingzhao, cell. Mol. Life Sci. 65:2431-44 (2008).
Production of toxin peptides is a complex process in venomous organisms, and is an even more complex process synthetically. Due to their conserved disulfide structures and need for efficient oxidative refolding, toxin peptides present challenges to synthesis. (See, Steiner and Bulaj, Optimization of oxidative folding methods for cysteine-rich peptides: a study of conotoxins containing three disulfide bridges, J. Pept. Sci. 17(1): 1-7, (2011); Góngora-Benitez et al., Optimized Fmoc solid-phase synthesis of the cysteine-rich peptide Linaclotide, Biopolymers Pept. Sci. 96(1):69-80, (2011)). Although toxin peptides have been used for years as highly selective pharmacological inhibitors of ion channels, the high cost of synthesis and refolding of the toxin peptides and their short half-life in vivo have impeded the pursuit of these peptides as a therapeutic modality. Far more effort has been expended to identify small molecule inhibitors as therapeutic antagonists of ion channels, than has been given the toxin peptides themselves. One exception is the approval of the small ω-conotoxin MVIIA peptide (Prialt®, ziconotide), an inhibitor of neuron-specific N-type voltage-sensitive calcium channels, for treatment of intractable pain. The synthetic and refolding production process for ziconotide, however, is costly and only results in a small peptide product with a very short half-life in vivo (about 4 hours).
A small clinical trial in humans showed that local, non-systemic injection of the non-peptide tetrodotoxin produced pain relief in patients suffering from pain due to cancer and/or to chemotherapy (Hagen et al., J Pain Symp Manag 34:171-182 (2007)). Tetrodotoxin is a non-CNS-penetrant inhibitor of sodium channels including NaV1.3 and NaV1.7; although it cannot be used systemically due to lack of selectivity among sodium channel subtypes, its efficacy provides further validation for treating chronic pain syndromes with inhibitors of NaV1.7 and/or NaV1.3 in peripheral neurons.
Polypeptides typically exhibit the advantage of greater target selectivity than is characteristic of small molecules. Non-CNS penetrant toxin peptides and peptide analogs selective for Nav1.7 are desired, and are provided by the present invention.